Number of recording tracks on a memory disk has been increased and track pitch has been narrowed to meet requirement of down-sizing and increasing storage capacity of the disk storage device. In association with this, more improvement in position control accuracy of the read/write head of the disk storage device has been required. To meet this improvement requirement, a so-called sector servo method has been widely employed. In the sector servo method, reference information including servo information is stored on the same memory surface of the disk for storing read/write data. The reference information is stored in a plurality of zones distributing along a circumferential direction of the disk between sectors for storing the read/write data. The sector provides a storage unit for storing data. The outline of the sector servo method is explained below referring to FIG. 2, though the method is well known.
FIG. 2(a) is an expanded view schematically showing a part of the disk surface in which three tracks T are shown. Reference information RI is stored in the disk and distributed along a circumferential direction of the disk which corresponds to the horizontal direction of FIG. 2(a). Sectors, as the data storage units are defined, are located on the tracks T between the stored reference information RI. As is well known, the reference information RI is stored first on the disk surface to define the tracks T and then the sectors are defined on each track by applying so-called formatting. FIG. 2(b) shows an expanded view of a portion cut out by two fine lines from FIG. 2(a) placing the reference information RI in its center. As shown in FIG. 2(b), the reference information RI consists usually of a front erasure section ER in which data is not stored, an identification section ID in which track address data is stored, and a servo information section SI for detecting a position of the read/write head. On the left hand side of the reference information RI, data Dg is stored on the tail portion of the sector S. On the right hand side of the reference information RI, a synchronous code Sy or the like is stored on the head portion of the sector S.
The narrow erasure section ER indicates that the identification section ID or the servo information section SI comes next. In the identification section ID, the track address data is stored by so-called Gray code so that the head may read the track address data even when the head is moving. In the servo information section SI, servo information is divided into four parts in the circumferential direction of the disk, usually designated as burst servo, which are displaced one by one in a predetermined manner in the radial direction of the disk, which corresponds to the vertical direction of FIG. 2(b). A reference information read out means, built in the disk storage device, detects the erasure section ER from an analog signal obtained by reading out the memory content by the read/write head 3. Then in synchronous with the detected erasure section ER, the reference information read out means reads out the identification section ID and the servo information section SI one by one. A head position control means of the disk storage device locates the head 3 on a designated track referring to the read out identification section ID and centers the head 3 on the track referring to the read out servo information section SI.
The disk storage device controls head position throughout its operation period by always reading out the reference information stored on the disk surface. However, the disk storage device, though rarely, fails to shift to read/write operation immediately after an electric power supply is connected to the disk storage device to start the device until the reference information read out means reaches its steady state of operation. This is because, in a transient period immediately after the disk storage device is started, the reference information read out means fails to read out reference information having the erasure section in its head portion, and so fails to identify the head position at the start of the disk storage device based on which the disk storage device conducts read/write operation. Factors which cause this failure in reading out the reference information in an early stage of starting the device will be explained below with reference to FIG. 3.
FIG. 3 shows wave forms of an analog read out signal RSa obtained by reading out the erasure section ER located in the head portion of the reference information RI and its vicinity shown in FIG. 2(a) by the head 3. FIG. 3(a) shows a normal wave form obtained when the reference information RI is successfully read out. In FIG. 3(a), the read out signal RSa shows zero amplitude in the portion corresponding to the erasure section ER. The amplitude of the read out signal RSa, which corresponds to the identification section ID or the data Dg located in the tail portion of the sector S which comes in advance to the reference information RI, does not keep zero amplitude though the signal oscillates between positives and negatives. In contrast to this, when the head 3 is positioned on a gap g between the tracks as shown by 3a in FIG. 2(b), the amplitude of the read out signal RSa tends to be small as shown in FIG. 3(b). In addition to this, the signal RSa may keep showing close to zero amplitude as indicated by Ef in FIG. 3(b), when unfavorable conditions accumulate, because the data Dg coming in advance to the erasure section ER may differ from a track to another as shown in FIG. 2(b).
Since the reference information read out means detects the position of the reference information RI by detecting the erasure section ER as described above, the reference information read out means tries to detect the reference information RI at an incorrect moment and fails in detection when a fake erasure section appears in the read out signal RSa as shown in FIG. 3(b). Probability of the mis-detection described above becomes higher as the position of the head 3 deviates more from the center of the track and when the disk storage device is started. At the start of the disk storage device, the head 3 is not always properly located. In the steady state operation of the disk storage device, one or two mis-readings out of the reference information may occur but they do not cause serious troubles, because the position of the head 3 is controlled by the head position control means and present position of the head 3 is always memorized.
In view of the foregoing, an object of the present invention is to provide a method of starting a disk storage device that prevents the device from failing to start caused by an error in reading out the reference information.